In FIG. 1 a crane vehicle is schematically illustrated which is put on support legs a serving as support element to safeguard the stand during operation. The support elements a are usually provided at the end of telescopic carriers b secured to the base frame c of the vehicle. At the site of the crane the entire vehicle is hydraulically lifted with the aid of the support elements. The vehicle wheels are not in contact with the ground.
When the forces transmitted vertically into the ground via the support elements, the weight distribution and, consequently, the center of gravity of the vehicle, in particular upon lifting a load, can be determined, and this is useful, in turn, to determine the tilting point of the crane.
One possibility of measuring the supporting forces consists in positioning a load cell in the flow of force of each support element. Said load cell then transmits a signal corresponding to the respective supporting force which is evaluated, for instance, in the afore-mentioned manner.
When lifting the vehicle by the support elements and when lifting a load, vehicle frames, telescopic carriers and support elements deform such that the ends or support legs of the support elements standing on the ground tend to move apart from each other in horizontal direction d, as illustrated in FIG. 2. This movement is partly prevented by the fact, however, that the friction coefficient between the ground and the support leg a is not zero. In this way horizontal reaction forces are formed between the support elements which generate bending moments in all components through which the flow of force is passed.
The problem in terms of measuring consists in the fact that any real load cell does not only react in the direction of force for which it is designed but more or less also to forces and moments in all other directions acting upon the same. For instance, in order to detect the position of the center of gravity of the vehicle or crane supported by the support elements merely the vertical forces, namely the supporting forces are relevant, however.
The horizontal forces occurring in the above-described application at the individual support elements amount to a maximum of 20% of the vertical forces to be measured.
The more distant the load cells are positioned from the end of the support legs on the ground, the more the bending moments increase due to the path of bending moments in the loaded parts. If load cells are arranged close to the ground-side end of the support elements, soiling of the support elements is particularly strong and might damage sensitive measuring instruments.